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Community and Q&A

Solar hot air preheat for HRV/ERV

Pat_Kiernan | Posted in Mechanicals on

Has anyone seen a successful approach to using a solar hot air collector to preheat the intake air for an HRV / ERV?

It would be fairly simple to bypass the collector when preheat wasn’t desirable.

One major concern is regulating temperature so excessive heat doesn’t compromise the HRV / ERV.

It seems like it could be an ideal way to temper incoming air on cold, clear Colorado days (climate zone 6B), and get a little extra solar gain.

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  1. heidner | | #1

    If the HRV/ERV will be pulling air from the south side of a building... and you have the space (and are willing), you might be able to pull the air through a solar hot air collector mounted on the outside wall.

    If you are worried about excessive heat - just blend in some of the normal bypass (outside air) and lower the temps going into the HRV/ERV. intake.

    The downside is space and the need for some active controls on a damper.

  2. GBA Editor
    Martin Holladay | | #2

    Your idea is frequently used to ventilate commercial buildings -- especially commercial buildings that require higher than usual levels of ventilation air.

    One company serving the needs of this type of building is SolarWall.

    Studies have shown that this approach isn't cost-effective for single-family homes. In other words, the energy savings are too low to justify the capital cost of the required equipment.

  3. Pat_Kiernan | | #3

    Dennis -- Thanks, I do have space on the south wall, though I'm still getting used to the aesthetics of mounting a collector.

    Martin -- Thanks also. I was hoping someone had come up with a cost effective approach for a very simple technology. I would appreciate any links to those studies if they are easy to find.

  4. GBA Editor
    Martin Holladay | | #4

    I'm not aware of any studies that look at your specific proposal.

    However, I have analyzed related proposals -- for example, the use of a glycol ground loop to preheat ventilation air. According to the best available data, that approach has a payback period of 4,400 years. For more information, see Using a Glycol Ground Loop to Condition Ventilation Air.

    I've also looked into solar hot air collectors that sell for about $1,600, and I concluded that these toys probably waste almost as much energy as they gather. For more information, see Solar Hot Air Collectors.

    However, you can do your own research if you want. Keep track of your materials cost and the number of hours of labor required to perform the work, and install enough monitoring equipment to determine your level of energy savings. Then report back to GBA. My prediction is that the energy savings will be low, and the hardware costs will be high.

  5. charlie_sullivan | | #5

    I think that combing solar hot air collectors with HRV/ERV would be worse than using them independently. The advantage of HRV over simple ventilation is that it warms the air for you. You don't need something else to warm it. If you collect 1000 BTU with the solar collector, and you use that air for the intake of an 85% efficient HRV, your net gain is only 150 BTU. If the solar collection is not quite cost effective used directly to gain 1000 BTU, it's nowhere near cost effective if you are only gaining 150 BTU.

    There's also the need for defrosting the HRV, but when you need that most is unlikely to coincide with when you have sunlight.

  6. Pat_Kiernan | | #6

    I was hoping to reduce electricity use for defrosting the HRV, but a hot air system would be useful for 4 to 6 hours per day, at best, in the dead of winter. Unfortunately, free heat isn't free to collect and control.

    Important point in the Solar Hot Air Collectors article -- watch for the risk of VOCs in home-brewed hot air collectors.

    I'll keep this on my radar and see if anything emerges that is worth experimenting with.

    Thanks, Martin and Charlie

  7. pacificstart | | #7

    I am also entertaining the idea of a DIY solar air furnace and the use of the existing HRV ductwork to distribute the air inside the house.
    I believe there are a few very important points that need to be taken into consideration:
    The cost of a DIY solar air collector should not exceed a couple hundred $ for a 4'x8' panel - based on my rough calculations anyway. There are plenty of examples on to build one on youtube, etc. The risk of VOC's is really worth paying attention to since these DIY builds may use all kinds of glue and plastic that is not made for use in hot air ventilation systems.
    The solar heated air should be brought in via an HRV bypass mechanism. Zehnder has that for example for bringing cool air during summer nights. But if the hot air comes in through the HRV it will be tempered down before being distributed so that defeats the purpose.
    The air flow should be somewhat increased during the 4-6 hours of winter day when the system is running in order to capture and distribute as much heat as possible. The higher the CFM, the less efficient the solar panel will be so more than one panel may be needed. Depending on the temperature of the hot air entering the building, the type of duct used, etc, the calculations can get complicated.
    This kind of system would work only for very air tight and very well insulated homes. The idea is to capture the heat from outside and store it inside the house. Open windows and doors would ruin the equation.
    Depending on where you are geographically - outside temperature and cloudiness will have a major impact on the viability of such system.
    The overall efficiency would be affected by the energy consumed by the fan used to push the air in the building. That could be further optimized by a small PV panel used to power the fan itself.

    Please let me know if I'm missing anything.

  8. GBA Editor
    Martin Holladay | | #8

    Your dreams are good, and are similar to the dreams of tinkerers who built solar hot air collectors in the 1970s. We've learned a lot since then. Decades of tinkering have relegated solar hot air collectors to the "quaint relic" section of crowded garages, where we store our old 8-track-tape players.

    When the sun shines, a good house doesn't need any heat.

    There isn't much sunshine during the months when homes need heat.

    Ducts that connect these collectors to your house tend to introduce air leaks.

    The energy gathered by these devices isn't worth enough to justify the investment in equipment to gather it.

    But the devices can be fun to build, even if they don't heat your house.

    For more information, see Solar Hot Air Collectors.

  9. Pat_Kiernan | | #9


    It sounds like you've given this some careful thought. I'm more interested in HRV preheat than general space heating, and I like the idea of increasing the ventilation rate when the collectors are warm. The HRV bypass may also prove useful.

    One of the factors that drive cost is that air is an inefficient way to collect, store and move heat, compared to water.

    Martin, you said "There isn't much sunshine during the months when homes need heat." Tha'st more true in the NE than in the SW USA. Certainly the days here are shorter in winter, but we still get many clear, sunny days in the winter in Colorado.

  10. GBA Editor
    Martin Holladay | | #10

    If you live in a well-designed house, with unshaded south-facing windows, you don't need any supplemental heat on "clear, sunny days in the winter in Colorado." You need the heat beginning at 4:00 p.m. or 5:00 p.m., after the sun has set.

  11. Pat_Kiernan | | #11

    Obviously, and if the house is also super-insulated, I shouldn't need heat until a while after the sun has set.

    But on a clear, sunny, 10 degree day, will the HRV require some heat to keep the core from freezing/frosting?

  12. GBA Editor
    Martin Holladay | | #12

    See this article: Zehnder Comfoair 350: Evaluation for Use in a Cold Climate.

    The authors imply that some type of preheat strategy or defrost strategy is necessary for HRVs when the exterior temperature drops to 23°F.

  13. Pat_Kiernan | | #13


    Thanks, that is a helpful paper along with your GBA post (also referenced in the paper)

    That GBA post is what got me started thinking about ways to minimize electrical use for HRV preheat. Thus, this Q&A thread.

  14. PAUL KUENN | | #14

    Zehnder schmender!

    Lunos is the way to go:) We have high humidity at -20F blowing with high winds of the Great Lakes and our Lunos HRVs do not frost up and the house remains very healthy and dry. I think they use 15watts max per pair. No duct work needed. Just a thought... PK

  15. Pat_Kiernan | | #15


    Thanks for the Lunos report. I'm glad the Lunos are working well for you. The house I'm designing is meant to accommodate cohouseholding (See -- a shared household with a mix of private and communal spaces. So I would need multiple Lunos pairs to make it work, I believe. That seems to get rather expensive.

  16. user-5740687 | | #16

    Pat, there is an excellent series on YouTube discussing HRV preheat and net zero construction in anchorage :

    They use Zender groundloop preheat to keep HRV delta temps at less than 2 degrees. Given that they build super tight houses, moderation of humidity and fresh air is a big issue at -35C sustained conditions.

    At very cold temperatures, the study Martin linked confirms that fresh air is drastically reduced as HRV spend very little time actually exchanging air. This is where the ground loop preheat makes a big difference in a net zero envelope.

    We are in zone 4, so regularly see -30C in winter. I also live in an extensively retrofitted older home. The irony of cold temp ventilation I've observed using Co2 monitoring in our home is this: When it's cold and windy, both stack effect and pressure differential on the home provide CO2 values in the 600-800 ppm range with four occupants and approx 2400 sq/ft conditioned space. Therefore, the automation system stops the Panasonic ERV under those conditions. This ERV has no defrost strategy, resorting to limited exchange times and low speed exhaust strategies when temps dip below -6C. During warmer periods, the ERV needs to run continuously at "high" speed (40CFM) to keep the home in the 800-1000 CO2 ppm range.

    I've researched the preheat strategy a great deal and concluded it would be a fun project, but given our envelope, would have low value. We also designed a 9000 sq/ft net zero target building for our business, also now using CO2 sensors to actively manage much larger Venmar commercial units. Co2 monitoring IMHO should be the next residential automation target..particularly as Ecobee stats can already manage limited summer cooling and HRV management. This in turn would provide better data for the preheat HRV value for a given envelope. wrote up a nice little blog post on our commercial HRV management strategy:

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